Chromosome disjunction during meiosis relies critically on associations that are generated by interhomolog crossover recombination events. In budding yeast and other organisms, the mismatch repair protein complexes Msh4/Msh5 and Mlh1/Mlh3 are required for the resolution of a large proportion of crossovers, along with proteins that assemble a striking proteinaceous chromosomal structure, the synaptonemal complex (SC). In budding yeast, Zip1 is a component of the transverse filaments of the tripartite SC structure, which lies at the interface of lengthwise-aligned homologous chromosomes. The relationship between SC proteins, the full- length SC structure and crossover maturation is unclear. Our recent studies involving an interspecies complementation experiment have revealed, somewhat surprisingly, that an ancestral version of Zip1 has the capacity to promote crossovers in budding yeast that are dependent on other budding yeast SC proteins and on Mlh3, but independent of the full-length SC, and independent of Msh4 function. These data have turned our attention toward studies that will identify the molecular features of Zip1 and the other SC proteins that mediate crossover maturation, and toward experiments that will characterize SC- and Msh4-independent, class I crossover events. Our proposed experiments employ straightforward genetic, biochemical, and microscopy approaches to 1) further characterize the crossover formation that occurs in the context of a crossover- proficient, synapsis-defective zip1 allele, 2) identify residues of Zip1 and other synapsis proteins that are important for a crossover function, and 3) characterize the structure and dynamics of SC proteins within the synapsis initiation complex structures formed at crossover sites. The high-throughput screening approaches that we employ are well matched to the expertise of the lab and its access to numerous advanced undergraduate students. The sum of our studies will contribute important details to our understanding of the structures and molecular mechanisms (including insight into possible ancestral mechanisms) that promote proper homolog disjunction in yeast. We expect these Aims to contribute to multiple high quality research papers, authored by two undergraduates and a doctoral student, which will be relevant to the broad meiosis and chromosome dynamics fields.